A study reviewing a decade of chickenpox prevention in the United States found that vaccination has resulted in a dramatic 90 per cent reduction in the disease, but even though the coverage was high, the single dose system did not confer sufficient immunity to stop the disease spreading. Experts are backing the 2-dose system that was introduced in 2006.

The study was the work of researchers from the Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia, and from Tufts University School of Medicine, in Boston, Massachusetts, and was published in the 1st September online issue of the journal Pediatrics.

The US was the first country in 1995 to bring in a single-dose chickenpox (varicella) vaccination program for all children. This policy changed to a two-dose system in 2006, with a top up dose for older children to catch up, because it was believed that the single-dose system would not be enough to prevent the spread of the disease.

The purpose of the study was to review a decade of experience with the single-dose system, and what evidence there might be contrasting the single and double dose method, in order to inform and identify challenges for policy makers.

The CDC and Tufts researchers reviewed the relevant published evidence on “varicella,” “varicella vaccine,” and “herpes zoster” (commonly known as “shingles“) and found that for the period 1995 to 2006:

  • The single dose chickenpox vaccine was 80 to 85 per cent effective at preventing the disease, whether mild or severe, and over 95 per cent effective at preventing the severe form.
  • The single dose vaccination had an excellent safety profile.
  • The single dose vaccination program reduced disease incidence by 90 per cent, hospitalizations by 88 per cent, and deaths by over 74 per cent.
  • The effect on costs was equally dramatic, showing a 74 per cent reduction in direct inpatient and outpatient medical expenditures over the period.
  • The rapid decline in cases plateaued from 2003 to 2006, and outbreaks still occurred, even among school populations with a high percentage of vaccinations.
  • Clinical trial evidence showed that compared with children given the single dose, those who received 2 doses developed higher immunity to the disease (as measured by antibody titers).
  • The figures showed the 2 dose children had a 3.3 lower risk of breakthrough disease.
  • The evidence for herpes zoster was not so clear cut, with 2 studies showing there was no overall increase in incidence and two studies showing there was.

The authors concluded that:

“A decade of varicella [chickenpox] prevention in the United States has resulted in a dramatic decline in disease; however, even with high vaccination coverage, the effectiveness of 1 dose of vaccine did not generate sufficient population immunity to prevent community transmission. “

ABC news reported that experts are concerned that many parents don’t take chickenpox seriously enough and there is a lack of awareness of just how many deaths there used to be, around two hundred per year, from the disease.

There is still a culture of the “chickenpox sleepover” among some parents, where news that one child has chickenpox causes them to place their non-infected children in close contact with the infected child so they become infected in a “planned” way.

Mark Slifka, associate scientist at the Vaccine and Gene Therapy Institute at the Oregon Health and Science University, told ABC this was “irresponsible” and parents would do better to get their children vaccinated:

“The vaccine is a vastly weaker strain of the same virus, and therefore much safer and with fewer side effects or disease complications,” said Slifka.

Even though they are unlikely to end up in hospital, and the risk of death is very low, what is the point of making your children suffer the discomfort of the full blown illness when the vaccine is safe and only gives them a mild experience of the disease? That is the message the experts are trying to get across to parents now, and close the transmission “gap” for good.

“Varicella Prevention in the United States: A Review of Successes and Challenges.”
Marin, Mona, Meissner, H. Cody, Seward, Jane F.
Pediatrics, Vol. 122 No. 3, e744-e751, Published online September 1, 2008.

Click here for Abstract.

Sources: Journal article, ABC.

Written by: Catharine Paddock, PhD